Twitter has added the ability to schedule tweets from the web version of the platform, on twitter.com today. This feature might come to its iOS and Android at some stage, but for now it is live on the web. Within the compose window, a new calendar icon has appeared from there you can go on the schedule the Tweet in the future.
Without this addition, if you needed to schedule a Tweet, third party solutions such as Buffer, TweetDeck, Hootsuite or other systems would have to be used.
— Twitter Support (@TwitterSupport) May 28, 2020